In recent years, attention has focused on a technique for making a thin film transistor (TFT) by using a semiconductor thin film (having a thickness of approximately several to several hundreds of nanometers) formed over a substrate having an insulating surface. The thin film transistors are widely applied to electronic devices such as ICs and electro-optical devices, and their rapid development as switching elements for image display devices is particularly desired.
Formation of switching elements for an image display device needs a high-accurate photolithography technique which can arrange the switching elements with high area efficiency, in order to display high-definition images. A large one-shot exposure apparatus, a stepper exposure apparatus, or the like is used in order to form switching elements with precision over a large-area substrate.
Although a large one-shot exposure apparatus can expose a large area to light at one time, there is a problem in that variation in illuminance intensity or degree of parallelization is large. Accordingly, a stepper exposure apparatus which uses an optical system is often used.
A region which is exposed to light at one time with the stepper exposure apparatus is limited. When light exposure is performed on an area which is larger than that region, several shots of light exposure are needed.
A semiconductor substrate called a silicon-on-insulator (SOI substrate) that has a thin single-crystal semiconductor layer on an insulating layer has been developed instead of a silicon wafer that is manufactured by thinly slicing an ingot of a single-crystal semiconductor. The SOI substrates are spreading as substrates in manufacturing microprocessors or the like. This is because an integrated circuit using an SOI substrate attracts attention as an integrated circuit in which parasitic capacitance between a drain of a transistor and a substrate can be reduced, performance of the semiconductor integrated circuit can be improved, and low power consumption is achieved.
As a method for manufacturing SOI substrates, a hydrogen ion implantation separation method is known (for example, see Patent Document 1: U.S. Pat. No. 6,372,609). The hydrogen ion implantation separation method is a method by which hydrogen ions are implanted into a silicon wafer to form a microbubble layer at a predetermined depth from the surface, and a thin silicon layer (SOI layer) is bonded to another silicon wafer using the microbubble layer as a cleavage plane. In addition to heat treatment for separating an SOI layer, it is necessary to perform heat treatment in an oxidizing atmosphere to form an oxide film on the SOI layer, remove the oxide film, and perform heat treatment at 1000° C. to 1300° C. in a reducing atmosphere to increase bonding strength.
On the other hand, attempts have been made to form an SOI layer on an insulating substrate such as glass. As an example of SOI substrates in which SOI layers are formed on glass substrates, an SOI substrate in which a thin single-crystal silicon layer is formed over a glass substrate having a coating film by a hydrogen ion implantation separation method is known (see Patent Document 2: U.S. Pat. No. 7,119,365). In this case also, a thin silicon layer (SOI layer) is formed on the glass substrate in such a way that a microbubble layer is formed at a predetermined depth from the surface by implantation of hydrogen ions to a single-crystal silicon wafer, the glass substrate and the single-crystalline silicon wafer are bonded, and the silicon wafer is separated using the microbubble layer as a cleavage plane.